Organic Field-Effect Transistors (OFETs) · Organic Field-Effect Transistors (OFETs) Gilles...

Organic Field-Effect Transistors (OFETs)

Gilles HorowitzCNRS - LPICM, Ecole Polytechnique, Palaiseau,

[email protected]

Organic Electronic Workshop, London, September 16, 2013

Organic field-effect transistor

2

VG

VD < VGVD > VG

semiconductor

insulator

Drain voltage

Dra

in c

urre

nt

linear

saturationpinch off

source drain

gate


Current-voltage curves

3

ID =WµCi

L

(VG � VT )VD � V 2

D

2

�

IDsat =WµCi

2L(VG � VT )

2

linear regime

saturation regime

output curves transfer curve


Transistors: What for?

4

❖A transistor is basically a ultra-fast electrical switching device✦Electronic circuits✦Display backplane✦Sensors


❖ Flexible display backplane from Plastic Logic, Cambridge UK

❖ Up to 1M transistors !


Organic materials ↔ carbon

6

1s2 2s2 2p2

divalent tetravalent

2s

2p

1s

methane CH4

sp3 hybridisation


Hybridisation sp2

7

pz

sp2 hybridisation1s2 2s2 2p2

2s

2p

1s

ethylene C2H2


Ethylene

8


Benzene

9


Poly(paraphenylenevinylene) (PPV)

10


Conjugated materials

11

Polyacetylene

Polyphenylene

Polythiophene

Poly(phenylene-vynilene)

Phthalocyanine(M=H, metal)

Oligothiophene (n=3…8)

Polyacene (n=2…5)

Polymers Small molecules


π-stacking

12

❖ Charge transport in a solid is limited by inter-molecular processes

❖ Highest mobility is along the direction perpendicular to π-stacking

Monolayer polyth iophene films have been previouslydeposited by the Langmuir-Blodget t technique,11,12 andby self-assembly on solu t ion /HOPG inter face.13 The fir stLB FET used regiorandom poly-(3-a lkylth iophene)s (RI-PAT) as the semiconduct ing mater ia l.14 The randomposit ion of the side cha ins in the RI-PAT inhibit s the !-!stacking and reduces the crysta llin ity of the film. Con-sequent ly, the field-effect mobility in the RI-PAT FET islow, 10-7-10-4 cm2/(V s). Amphiphilic PAT form stableLB films with la rge ordered domains a t the a ir-waterinterface.15 RR-P3HT is also likely to form stable Langmuirfilms due to it s ha iry-rodlike polymer st ructure.16,17However , the films formed were r igid and st iff and a water-soluble amphiphile spread-a iding compound was in t ro-duced in order to produce homogeneous and flexible films.The t ransfer red LB films were or ien ted with the polymerbackbone para llel to the subst ra te in an edge-on confor -mat ion , which is favorable for the opera t ion of a FET. Theabsolute thickness of a single LB layer was not determineddirect ly and was est imated to conta in a double molecula rlayer .17Monolayers of highly regioregular oligo-alkylthiophenes

were a lso prepared through self-assembly onto a solu t ion /HOPG inter face.13 The a lkyl side cha ins are orderedepitaxia lly on the HOPG st ructure forcing the rodlikesegments to lay fla t on the sur face. The rodlike segmentsare ordered in a para llel lamella -type stack with in ter -digita ted a lkyl side cha ins to achieve maximum van derWaals in teract ions. Monolayers are a lso observed forR-subst itu ted18 and "-subst itu ted19 oligoth iophenes whilemult ilayers are very rarely seen and a lso only for near -sa tura ted solu t ions.In th is paper , u lt ra th in RR-P3HT films are deposited

through a dip-coat ing technique. In dip-coat ing the system

is near equilibr ium and the na ture of the subst ra te/polymer in ter face st rongly influences the film format ion .This techniqe provides fast and simple means for theprepara t ion of ult ra th in and undoped films in a cont rolleda tmosphere. Moreover , th is technique combines thetendency of the polymer to form stable monolayers a t thesolu t ion /subst ra te in ter face as observed on HOPG, withthe stability of the monolayer in a ir , as observed for theLB films.We studied the fabr ica t ion and per formance of a

polyth iophene monolayer -th ick FET. By proper select ionof the subst ra te and cont rol of the solu t ion concent ra t ion ,self-organized monolayers of RR-P3HT are depositedthrough dip coat ing. The growth mechanism and proper-t ies of the films were studied in var ious techniquesincluding AFM, X-ray, and UV-vis absorpt ion . The filmsare u t ilized as the semiconduct ing component in FETs.Monolayer FETs provide a direct exper imenta l system tostudy the charge t ranspor t a t the charge accumula t ionlayer .

Experimental SectionMaterials and Film Deposition .Ultrathin films ofRR-P3HT

were prepared by dipping hydrophilic Si/SiO2 (oxide th ickness2000 Å) and glass subst ra tes in to a dilu te solu t ion of RR-P3HT(McCullough route, MA ) 80k) in xylene (Aldr ich 99%, anhy-drous). The concent ra t ion of the solu t ion was <0.5 mg/mL. Thesubst ra te was in t roduced ver t ica lly in to the RR-P3HT solu t ionand withdrawn after 1 min when it was left to dry ver t ica lly. Thefilms were annea led for 10 h in a vacuum (3 ! 10-6 mbar) a t 100°C. Mult ilayer st ructures were a t tempted by repea t ing the dipsequence and dip/annea l sequence severa l t imes.Thin Film Characterization . The topography, morphology,

and th ickness of the RR-P3HT films on glass and Si/SiO2 werestudied using a Digita l Inst ruments nanoprobe a tomic forcemicroscope in tapping mode. The th ickness was measured byscanning the edge of a mechanica l scra tch in the film. Thistechnique enables to obta in average th ickness va lues overrela t ively la rge areas of the sur face. The films deposited on glasswere a lso used for UV-vis absorpt ion , which was recorded ona Hewlet t -Packard 8453 spect rophotometer .The st ructure of the dipcast films was character ized by both

X-ray reflect ivity and grazing incidence diffract ion a t the BW2z-axis diffractometer a t the synchrot ron radia t ion facility,HASYLAB at DESY in Hamburg, and by X-ray reflect ivity a t aRigaku 18 kW rota t ing Cu anode X-ray genera tor . The radia t ionfrom the rotat ing anode was monochromatized by a Ge(111) singlecrysta l and collimated by a high-precision slit system for accuratecont rol of the beam condit ions.The high br illiance of the synchrot ron X-ray source made it

possible to observe diffract ion from self-organized domains withinthe th in films. The samples were mounted in a capton chamber ,flooded by He, thereby minimizing a ir sca t ter ing and radia t iondamage. Using a fixed, grazing incidence angle above the cr it ica langle for tota l reflect ion from the film, but below the cr it ica langle for the subst ra te, the sca t ter ing from the subst ra te isreduced rela t ive to the sca t ter ing of the film. At a photon energyof 10 keV, the cr it ica l angle of the subst ra te (Si) is 0.1797°, andthe cr it ica l angle for the film is "0.123°. The sample normal waskept in a ver t ica l plane conta in ing the inciden t X-ray beam,providing the opt imum flux a t the sample while main ta in ing ahigh resolu t ion of the incidence angle. By t ransla t ing the samplehor izonta lly, the diffract ion signa l can be probed as funct ion ofposit ion , a llowing us to ident ify regions with differen t degreesof order ing. The grazing incidence diffract ion spect ra werecor rected for geomet r ic and polar iza t ion effect s.20,21Reflect ivit ies were ca lcu la ted from models of the dist r ibu t ion

of mass density a long the sample normal. The density profileswere parametr ized into three slabs corresponding to the chemicalcomposit ion of P3HT and a slab of SiO2 on top of the Si bulkcrysta l. The ext ra slabs in t roduce addit iona l degrees of freedom

(11) Wegner , G. Thin S olid Film s 1992, 216, 105.(12) Reitzel, N.; Greve, D. R.; Kjaer , K.; Howes, P . B.; J ayamaram,

M.; Savoy, S.; McCullough, R. D.; McDevit t , J . T.; Bjornholm, T. J . Am .Chem . S oc. 2000, 122, 5877.(13) Kirschbaum, T.; Azumi, R.; Mena-Oster itz, E .; Bauer le, P . New

J . Chem . 1999, 23, 241.(14) Pa loheimo, J .; Kuiva la inen , P .; Stubb, H.; Vuor imaa , E .; Yli-

Laht i, P . Appl. Phys. Lett. 1990, 56, 1157.(15) Bøggild, P .; Rey, F .; Hassenkam, T.; Greve, D. R.; Bjornholm,

T. Adv. Mater. 2000, 12, 947.(16) Ochia i, K.; Tabuchi, Y.; Rikukawa, M.; Sanui, K.; Ogata , N.

Thin S olid Film s 1998, 327, 454.(17) Xu, G.; Bao, Z.; Groves, J . T. Langmuir 2000, 16, 1834.(18) Azumi, R.; Gotz, G.; Bauer le, P . S ynth . Met. 1999, 101, 569.(19) Azumi, R.; Gotz G.; Debaerdemaeker , T.; Bauer le, P .Chem . Eur.

J . 2000, 6, 735.(20) Feidenhansl, R. S urf. S ci. Rep. 1989, 10, 105.(21) Vlieg, E . J . Appl. Crystallogr. 1997, 30, 532.

Figure 1. (a ) The prefer red or ien ta t ion of RR-P3HT in a th infilm transistor is with the !-! stacking para llel to the subst ra teand (b) the molecula r st ructure of RR-P3HT.

Regioregular Field-Effect T ransistors Langm uir, Vol. 18, No. 26, 2002 10177


High mobility «push-pull» polymers

13

Li, J. et al., Sci. Rep. 2012, 2, 754

acceptor

donor

❖ Strong donnor - weak acceptor → p-channel

❖ Weak donnor - strong acceptor → n-channel


Contact resistance

❖Contact resistance may substancially reduce the output current of the resistance

❖The «effective» field-effect mobility is a combination of the «intrinsic» mobility of the organic semiconductor and the contact resistance


Origin of the contact resistance

❖ Injection barrier height. The highest the barrier, the highest the contact resistance

❖ Morphology of the semiconductor layer. Because the electrode and semiconductor surfaces may have different surface energy, the morphology of the organic semiconductor film may have different morphological structure, thus leading to disorderd interfacial layer

16


Barrier height

17

HOMO

LUMO

vacuum level

electron barrier

hole barrier

∆


Electrolyte-gated OFET

18

5376 J. Am. Chem. SOC., Vol. 106, No. 18, 1984 i r- C ountereleclrode

R ference

Polypyrrote

S o y e / Drajn

Communications to the Editor

fixed gate potentials, VG. When VG is held at a negative potential where the polypyrrole is expected to be insulating, the device is "off'' and I D is small a t values of VD < 0.5 V. When VG is moved to potentials more positive than the oxidation potential of polypyrrole, - 4 . 2 V vs. SCE, the device "turns on" and a significant steady-state value of ID can be observed for modest values of VD. We take the redox potential of polypyrrole to approximate the value of V,, the gate potential a t which the device starts to turn on. For VG more positive than VT the value of ID increases a t a given value of VD, in a manner consistent with increasing conductivity of polypyrrole with an increasing degree of oxidation. At sufficiently positive values of V G , >,+OS V vs. SCE, ID becomes insensitive to further positive movement of VG at a given value of VD, a result consistent with measurements6 of the resistance of the oxidized polypyrrole coated on a microelectrode array. We typically use only a small range of VD values because we seek to minimize electrochemical reactions at the source/polymer and drain/polymer interfaces. All measurements are for deoxygenated solutions, owing to problems stemming from irreversible oxidation of the polymer. However, in the absence of O2 good durability (several days of use) can be achieved. Shelf life in the dry state exceeds 1 week for derivatized arrays.

For the devices studied, some fraction of lo-* C of charge is required to obtain the maximum steady-state value of ID at VD = 0.2 V; the value of ID achievable with the device represented in Figure 1 is -4 X C/s. Thus, a small signal to the gate can be amplified in much the same way that a small electrical signal can be amplified with a solid-state transistor.' A major diference, of course, between the solid-state device and the molecule-based device is that the turn on/turn off time in the molecule-based system depends on the rate of a chemical reaction whereas no chemical reactions take place in the solid-state devices. For the devices fabricated the on-off time is of the order of 10 s; the curves in Figure 2 are steady-state curves. For the molecule-based system the properties such as VT and minimum turn on signal can be adjusted with rational variation in the monomer used to prepare the polymer. Smaller dimensions can lead to faster switching times and different molecule-based gate materials may have superior rates of switching compared to polypyrrole.

The molecule-based transistor reported here has no immediate practical application. The fundamental accomplishment is the demonstration of the synthesis of an interfacial chemical system that has a specific function. There is presently considerable interest in interfacing microelectronic devices with chemical and biological systems for sensor applicationss and in "molecular electronics" in generaL8 Our work establishes that principles learned from the study of polymer-modified macroscopic electrode^^^^-'^ can

I sto, I

S I S u b s t r a t e I Ly9 -ID - - I 0

Figure 1. Cross-sectional view of the device fabricated and representation of the circuit elements used to characterize it. The Si02 layer is -0.45 pm thick and is on a -0.3 mm thick (100) Si substrate. The source, gate, and drain are Au, - 3 pm wide X 140 pm long X 0.12 pm thick coated with -lo-' mol/cm2 of polypyrrole. When characterized, the derivatized microelectrode array, counter, and reference electrodes are immersed in electrolyte solution.

35 / " I " ' '

v : O B V v s S C E Polypyrro le-Based Trans is tor Output Character is t ics C H 3 C N I O I M [ n - B u 4 N ] CIO,

-0 1 v / -0 2--1 o v

01 0 2 VlJ ,v

Figure 2. Output characteristics of the transistor shown in Figure 1 in CH3CN/O. 1 M [ ~ z - B u ~ N I C I O ~ .

be electrically connected by the polymer.6 The experimental evidence for this is that a11 of the connected electrodes show the same cyclic voltammetry response (peak position, shape, and area), when measured individually and when they are externally connected together and driven as one electrode. When three microelectrodes are connected together with the polypyrrole as in Figure 1, the charge associated with the oxidation and reduction of the bound polymer is of the order of C/cm2 of exposed Au. Examination of derivatized microelectrode arrays by scanning electron microscopy confirms the presence of the polymer.

Characterization of a derivatized three-electrode array shows that a transistor characteristic can be obtained (Figure 2).' In this set of measurements the two outer electrodes are wired as source and drain and the middle electrode is the gate (Figure 1).3 Figure 2 shows the current between source and drain, ID, as a function of the potential between source and drain, VD, at various

~~

(6) Kittlesen, G. P.; White, H. S . ; Wrighton, M. S . J . Am. Chem. SOC., in press. [This article is a full account of the procedure used to fabricate, characterize, and derivatize the microelectrodes used in this work.]

(7) A 3-V battery and voltage divider were used to vary VD from 0.0 to 0.2 V. The gate electrode was connected in a conventional three-electrode potentiostatic arrangement employing a Pine Model RDE 3 bipotentiostat. A Pt wire and saturated calomel electrode, SCE, were used as the counter and reference electrodes, respectively. ID was measured by recording the potential drop across a 100-Q resistor resulting from the steady-state current between the source and drain electrode. All curves are for the microelectrode array and Pt and SCE electrodes immersed in CH,CN/O.l M [n-Bu4N]C104 at 25 OC under N2.

(8) See, for example, the special issue on "Molecular Electronics": IEE Proc. I1983, 103, 197-263.

(9) (a) Murray, R. W. Acc. Chem. Res. 1980, 13, 135-141. (b) Facci, J.; Murray, R. W. J . Electroanal. Chem. 1981, 124, 339-342. (c) Daum, P.; Murray, R. W. J . Electroana/. Chem. 1979, 103, 289-294. (d) Daum, P.; Lenhard, J. R.; Rolison, D. R.; Murray, R. W. J . Am. Chem. SOC. 1980, 102, 4649-4653.

(IO) (a) Peerce, P. J.; Bard, A. J . J . E/ectroana[. Chem. 1980, 108, 121-125. (b) Rubenstein, I.; Bard, A. J. J . Am. Chem. SOC. 1981, 103, 512-515. (c) Abruna, H. D.; Bard, A. J. J . Am. Chem. SOC. 1982, 104, 2641-2645. (d) Henning, T. P. White, H. S.; Bard, A. J . J . Am. Chem. SOC. 1981,103,3937-3941. (e) Daum, P.; Murray, R. W. J. Phys. Chem. 1981, 85, 389-396.

(11) (a) Kaufman, F. G.; Schroeder, A. H.; Engler, A. H.; Kramer, S . R.; Chambers, J . B. J . Am. Chem. SOC. 1980, 102,483-488. (b) Kaufman, F. B.; Engler, E. M. J . Am. Chem. SOC. 1979, 101, 547-549. (c) Lau, A. N. K.; Miller, L. L. J. Am. Chem. SOC. 1983,105, 5271-5277. (d) Lau, A. N. K.; Miller, L. L.; Zinger, B. J . Am. Chem. SOC. 1983, 105, 5278-5284. (e) Van DeMark, M. R.; Miller, L. L. J. Am. Chem. Soc. 1978,100, 3223-3224. (0 Landrum, H. L.; Salmon, R. T.; Hawkridge, F. M. J . Am. Chem. SOC.

(12) (a) Tsou, Y.-M.; Anson, F. C. J . Electrochem. SOC. 1984, 131, 595-601. (b) Shigehara, K.; Oyama, N.; Anson, F. C. Inorg. Chem. 1981, 20, 518-522. (c) Shigehara, K.; Oyama, N.; Anson, F. C . J . Am. Chem. SOC. 1981, 103, 2552-2558. (d) Buttry, D. A,; Anson, F. C. J . Am. Chem. SOC. 1983,105,685-689. ( e ) Oyama, N.; Anson, F. C. J . Am. Chem. SOC. 1979, 101, 3450-3456.

1977, 99, 3154-3158.

5376 J. Am. Chem. SOC., Vol. 106, No. 18, 1984 i r- C ountereleclrode

R ference

Polypyrrote

S o y e / Drajn

Communications to the Editor

fixed gate potentials, VG. When VG is held at a negative potential where the polypyrrole is expected to be insulating, the device is "off'' and I D is small a t values of VD < 0.5 V. When VG is moved to potentials more positive than the oxidation potential of polypyrrole, - 4 . 2 V vs. SCE, the device "turns on" and a significant steady-state value of ID can be observed for modest values of VD. We take the redox potential of polypyrrole to approximate the value of V,, the gate potential a t which the device starts to turn on. For VG more positive than VT the value of ID increases a t a given value of VD, in a manner consistent with increasing conductivity of polypyrrole with an increasing degree of oxidation. At sufficiently positive values of V G , >,+OS V vs. SCE, ID becomes insensitive to further positive movement of VG at a given value of VD, a result consistent with measurements6 of the resistance of the oxidized polypyrrole coated on a microelectrode array. We typically use only a small range of VD values because we seek to minimize electrochemical reactions at the source/polymer and drain/polymer interfaces. All measurements are for deoxygenated solutions, owing to problems stemming from irreversible oxidation of the polymer. However, in the absence of O2 good durability (several days of use) can be achieved. Shelf life in the dry state exceeds 1 week for derivatized arrays.

For the devices studied, some fraction of lo-* C of charge is required to obtain the maximum steady-state value of ID at VD = 0.2 V; the value of ID achievable with the device represented in Figure 1 is -4 X C/s. Thus, a small signal to the gate can be amplified in much the same way that a small electrical signal can be amplified with a solid-state transistor.' A major diference, of course, between the solid-state device and the molecule-based device is that the turn on/turn off time in the molecule-based system depends on the rate of a chemical reaction whereas no chemical reactions take place in the solid-state devices. For the devices fabricated the on-off time is of the order of 10 s; the curves in Figure 2 are steady-state curves. For the molecule-based system the properties such as VT and minimum turn on signal can be adjusted with rational variation in the monomer used to prepare the polymer. Smaller dimensions can lead to faster switching times and different molecule-based gate materials may have superior rates of switching compared to polypyrrole.

The molecule-based transistor reported here has no immediate practical application. The fundamental accomplishment is the demonstration of the synthesis of an interfacial chemical system that has a specific function. There is presently considerable interest in interfacing microelectronic devices with chemical and biological systems for sensor applicationss and in "molecular electronics" in generaL8 Our work establishes that principles learned from the study of polymer-modified macroscopic electrode^^^^-'^ can

I sto, I

S I S u b s t r a t e I Ly9 -ID-- I 0

Figure 1. Cross-sectional view of the device fabricated and representation of the circuit elements used to characterize it. The Si02 layer is -0.45 pm thick and is on a -0.3 mm thick (100) Si substrate. The source, gate, and drain are Au, - 3 pm wide X 140 pm long X 0.12 pm thick coated with -lo-' mol/cm2 of polypyrrole. When characterized, the derivatized microelectrode array, counter, and reference electrodes are immersed in electrolyte solution.

35 / " I " ' '

v : O B V v s S C E Polypyrro le-Based Trans is tor Output Character is t ics C H 3 C N I O I M [ n - B u 4 N ] CIO,

-0 1 v / -0 2--1 o v

01 0 2 VlJ ,v

Figure 2. Output characteristics of the transistor shown in Figure 1 in CH3CN/O. 1 M [ ~ z - B u ~ N I C I O ~ .

be electrically connected by the polymer.6 The experimental evidence for this is that a11 of the connected electrodes show the same cyclic voltammetry response (peak position, shape, and area), when measured individually and when they are externally connected together and driven as one electrode. When three microelectrodes are connected together with the polypyrrole as in Figure 1, the charge associated with the oxidation and reduction of the bound polymer is of the order of C/cm2 of exposed Au. Examination of derivatized microelectrode arrays by scanning electron microscopy confirms the presence of the polymer.

Characterization of a derivatized three-electrode array shows that a transistor characteristic can be obtained (Figure 2).' In this set of measurements the two outer electrodes are wired as source and drain and the middle electrode is the gate (Figure 1).3 Figure 2 shows the current between source and drain, ID, as a function of the potential between source and drain, VD, at various

~~

(6) Kittlesen, G. P.; White, H. S . ; Wrighton, M. S . J . Am. Chem. SOC., in press. [This article is a full account of the procedure used to fabricate, characterize, and derivatize the microelectrodes used in this work.]

(7) A 3-V battery and voltage divider were used to vary VD from 0.0 to 0.2 V. The gate electrode was connected in a conventional three-electrode potentiostatic arrangement employing a Pine Model RDE 3 bipotentiostat. A Pt wire and saturated calomel electrode, SCE, were used as the counter and reference electrodes, respectively. ID was measured by recording the potential drop across a 100-Q resistor resulting from the steady-state current between the source and drain electrode. All curves are for the microelectrode array and Pt and SCE electrodes immersed in CH,CN/O.l M [n-Bu4N]C104 at 25 OC under N2.

(8) See, for example, the special issue on "Molecular Electronics": IEE Proc. I1983, 103, 197-263.

(9) (a) Murray, R. W. Acc. Chem. Res. 1980, 13, 135-141. (b) Facci, J.; Murray, R. W. J . Electroanal. Chem. 1981, 124, 339-342. (c) Daum, P.; Murray, R. W. J . Electroana/. Chem. 1979, 103, 289-294. (d) Daum, P.; Lenhard, J. R.; Rolison, D. R.; Murray, R. W. J . Am. Chem. SOC. 1980, 102, 4649-4653.

(IO) (a) Peerce, P. J.; Bard, A. J . J . E/ectroana[. Chem. 1980, 108, 121-125. (b) Rubenstein, I.; Bard, A. J. J . Am. Chem. SOC. 1981, 103, 512-515. (c) Abruna, H. D.; Bard, A. J. J . Am. Chem. SOC. 1982, 104, 2641-2645. (d) Henning, T. P. White, H. S.; Bard, A. J . J . Am. Chem. SOC. 1981,103,3937-3941. (e) Daum, P.; Murray, R. W. J. Phys. Chem. 1981, 85, 389-396.

(11) (a) Kaufman, F. G.; Schroeder, A. H.; Engler, A. H.; Kramer, S . R.; Chambers, J . B. J . Am. Chem. SOC. 1980, 102,483-488. (b) Kaufman, F. B.; Engler, E. M. J . Am. Chem. SOC. 1979, 101, 547-549. (c) Lau, A. N. K.; Miller, L. L. J. Am. Chem. SOC. 1983,105, 5271-5277. (d) Lau, A. N. K.; Miller, L. L.; Zinger, B. J . Am. Chem. SOC. 1983, 105, 5278-5284. (e) Van DeMark, M. R.; Miller, L. L. J. Am. Chem. Soc. 1978,100, 3223-3224. (0 Landrum, H. L.; Salmon, R. T.; Hawkridge, F. M. J . Am. Chem. SOC.

(12) (a) Tsou, Y.-M.; Anson, F. C. J . Electrochem. SOC. 1984, 131, 595-601. (b) Shigehara, K.; Oyama, N.; Anson, F. C. Inorg. Chem. 1981, 20, 518-522. (c) Shigehara, K.; Oyama, N.; Anson, F. C . J . Am. Chem. SOC. 1981, 103, 2552-2558. (d) Buttry, D. A,; Anson, F. C. J . Am. Chem. SOC. 1983,105,685-689. ( e ) Oyama, N.; Anson, F. C. J . Am. Chem. SOC. 1979, 101, 3450-3456.

1977, 99, 3154-3158.

White, HS; Kittlesen, GP; Wrighton, MS, Chemical derivatization of an array of three gold microelectrodes with polypyrrole: Fabrication of a molecule-based transistor. J. Am. Chem. Soc. (1984) 106 5375


❖ The organic electrochemical transistor (OECT) combines electronic (from source to drain) and ionic (doping-undoping process) transport

❖ Advantages✦ Ease of fabrication✦ Very low operating voltage

❖Drawbacks✦ Very long switching time (limited by ionic transport)


❖ Unlike all others conducting polymers PEDOT:PSS is conducting when undoped and become insulating when doped✦ Normally on transistors✦ PEDOT:PSS films easy to process from water suspension

20

Rebirth of the OECT. PEDOT:PSS

PEDOT PSS


Electrochemical devices

31

Figure 17. Schematic illustration of the three-terminal OECT. The gate electrode is indicated

with a G, the source and drain contacts are denoted with S and D respectively.

The characteristics of the three-terminal transistor are shown in Figure 18. In

the first quadrant (VD>0 and ID>0), the transistor behaves linearly at zero gate

voltage. When a positive gate voltage is applied, the channel is reduced, resulting in

increased impedance. As VD is increased, the channel is oxidised back to its low

impedance state as can be seen in Figure 18. In the third quadrant (VD<0 and ID<0)

the transistor shows saturation behaviour similar to the four-terminal transistor.

Figure 18. ID vs. VD characteristics, at various gate voltages for a three-terminal

electrochemical transistor. Note the similarity to I(V) characteristics for a depletion mode

MOSFET.

Electrochemical devices

31

Figure 17. Schematic illustration of the three-terminal OECT. The gate electrode is indicated

with a G, the source and drain contacts are denoted with S and D respectively.

The characteristics of the three-terminal transistor are shown in Figure 18. In

the first quadrant (VD>0 and ID>0), the transistor behaves linearly at zero gate

voltage. When a positive gate voltage is applied, the channel is reduced, resulting in

increased impedance. As VD is increased, the channel is oxidised back to its low

impedance state as can be seen in Figure 18. In the third quadrant (VD<0 and ID<0)

the transistor shows saturation behaviour similar to the four-terminal transistor.

Figure 18. ID vs. VD characteristics, at various gate voltages for a three-terminal

electrochemical transistor. Note the similarity to I(V) characteristics for a depletion mode

MOSFET.

• PEDOT:PSS may play the role of both a semiconductor and conducting electrodes

• The transistor structure is realized by laser ablation of PEDOT:PSS films on PET foils

Nilsson, D. et al., Adv. Mater. (2005) 17 353


Electrolyte-gated OFET vs OECT

and there have been extensive studies of inorganic semiconduc-tor-based ion-sensitive field-effect transistors (ISFETs)19-24as well as semiconducting polymer-based electrochemicaltransistors25-31 and ion sensors32-33 in the last 30 years.In this paper, we expand substantially on our earlier reports9-12

and demonstrate that the polymer electrolyte system consistingof polyethylene oxide (PEO) plus a lithium salt can be used asthe gate insulator in transistors employing a variety of polymersemiconductor films. Specifically, we show here that (i) solidpolymer electrolytes based on PEO can be used to gate OFETscomposed of three solution-processable polymer semiconduc-tors, enabling high-output currents at low gate voltages; (ii) theresults are general in that we can change the polymer semi-conductor and the dissolved ions in the electrolyte to obtainqualitatively similar results; (iii) the conductance maximumpreviously reported10-12 is also a generally observable phenom-enon for all three polymer semiconductors, independent of thespecific salt used; (iv) the induced charge density is substantial(!2 " 1014 charges/cm2 or !30 µC/cm2) and comparable forall polymer semiconductor systems examined; (v) quantitatively,there are important differences in the current-voltage charac-teristics between polymer electrolyte-gated OFETs (PEG-FETs)based on various semiconductors, yet the charge mobilities canbe exceptionally large; thus there is nothing about the polymerelectrolyte/semiconductor interface that precludes high mobili-ties; (vi) electrostatic charging (as opposed to electrochemicaldoping) can be the dominant operating mechanism in PEG-FETsunder vacuum conditions; and (vii) bias stress effects can besmall for PEG-FETs and are dependent on the polymersemiconductor employed. A schematic of the PEG-FET and anoptical image of a completed device are displayed in Figure 1.A key point of the current paper is the mechanism of transistor

action in PEG-FETs. In particular, an important distinction canbe made between charging Via the field-effect (as occurs inconventional metal-oxide-semiconductor field-effect transistors,MOSFETs) and electrochemical doping operational modes.34The critical difference between these two mechanisms is thepermeability of the semiconductor layer to the ions in thepolymer electrolyte, Figure 2. In an electrochemical transistor,25-31ions migrate into the semiconductor to stabilize charges in thepolymer semiconductor backbone; this constitutes an electro-chemical reaction, or doping process, which increases the filmconductivity. On the other hand, if ions do not penetrate into

the semiconductor during OFET operation, current modulationis achieved by the electrostatic induction of charge carriers inthe semiconductor in response to the high electric field at thepolymer semiconductor/polymer electrolyte interface (i.e., the“field-effect” is operative). In this case, an electric double layeris formed at the polymer semiconductor/polymer electrolyteinterface, Figure 2. These two scenarios represent limiting casesfor the degree of ion migration into the polymer semiconductor,and a combination of the two mechanisms has also been usedto explain OFET operation with an electrolyte gate.8 Here, wepresent experimental evidence demonstrating that PEG-FETscan be operated via the field-effect under vacuum conditions,

(18) Bergveld, P. IEEE Trans. Biomed. Eng. 1970, 17, 70-71.(19) Bergveld, P. IEEE Trans. Biomed. Eng. 1972, 19, 342-351.(20) Moss, S. D.; Janata, J.; Johnson, C. C. Anal. Chem. 1975, 47, 2238-2243.(21) Tahara, S.; Yoshii, M.; Oka, S. Chem. Lett. 1982, 3, 307-310.(22) Schloh, M. O.; Leventis, N.; Wrighton, M. S. J. Appl. Phys. 1989, 66,

965-968.(23) Natan, M. J.; Mallouk, T. E.; Wrighton, M. S. J. Phys. Chem. 1987, 91,

648-654.(24) Mariucci, L.; Fortunato, G.; Pecora, A.; Bearzotti, A.; Carelli, P.; Leoni,

R. Sens. Acutators, B 1992, B6, 29-33.(25) White, H. S.; Kittlesen, G. P.; Wrighton, M. S. J. Am. Chem. Soc. 1984,

106, 5375-5377.(26) Chao, S.; Wrighton, M. S. J. Am. Chem. Soc. 1987, 109, 6627-6631.(27) Chao, S.; Wrighton, M. S. J. Am. Chem. Soc. 1987, 109, 2197-2199.(28) Thackeray, J. W.; White, H. S.; Wrighton, M. S. J. Phys. Chem. 1985, 89,

5133-5140.(29) Ofer, D.; Crooks, R. M.; Wrighton, M. S. J. Am. Chem. Soc. 1990, 112,

7869-7879.(30) Ofer, D.; Wrighton, M. S. J. Am. Chem. Soc. 1988, 110, 4467-4468.(31) Tatischeff, H. B.; Fritsch-Faules, I.; Wrighton, M. S. J. Phys. Chem. 1993,

97, 2732-2739.(32) Marsella, M. J.; Carroll, P. J.; Swager, T. M. J. Am. Chem. Soc. 1995,

117, 9832-9841.(33) Zhu, S. S.; Carroll, P. J.; Swager, T. M. J. Am. Chem. Soc. 1996, 118,

8713-8714.(34) Lin, F.; Lonergan, M. C. Appl. Phys. Lett. 2006, 88, 133507.

Figure 1. (a) Polymer electrolyte-gated organic field-effect transistor (PEG-FET) schematic in cross-section (not drawn to scale). Source (IS), drain(ID), and gate (IG) currents were measured simultaneously while applyingthe source-gate (VG) and source-drain (VD) voltages. (b) Optical imageof a completed PEG-FET (top view); the source-drain separation (channellength) is 200 µm, and the channel width is 2000 µm.

Figure 2. Schematic cross-sections of a portion of a PEG-FET channelshowing the key difference in the extent of ion penetration between (a)electrostatic charging and (b) electrochemical doping as possible operatingmechanisms.

A R T I C L E S Panzer and Frisbie

6600 J. AM. CHEM. SOC. 9 VOL. 129, NO. 20, 2007

M. J. Panzer, C. D. Frisbie, J. Am. Chem. Soc. (2007) 129 6599


Biosensor

23

❖A biosensor is a device that converts a biological event into an electrical or optical signal

❖A biosensor includes the following elements✦A bioreceptor that identifies the species to be

detected✦A transducer that converts the biological

recognition event into a mesurable (electrical or optical) signal


EGOFET pH sensor

24

Electrolyte-gated organic field-effect transistors for sensing applicationsF. Buth, D. Kumar, M. Stutzmann, and J. A. Garridoa!

Walter Schottky Institut, Technische Universität München, Am Coulombwall 4, 85748 Garching, Germany

!Received 4 February 2011; accepted 31 March 2011; published online 15 April 2011"

We report on the electrolytic gating of !-sexithiophene thin film transistors, in which the organicsemiconductor is in direct contact with an electrolyte. Due to the large capacitance of the electricaldouble layer at the electrolyte/semiconductor interface, modulation of the channel conductivity viaan electrical field effect is achieved at low voltages. The transistors are stable for several hours andare sensitive to variations in the pH resulting from a pH-dependent surface charge, which modulatesthe threshold voltage. The response to different ion concentrations is described by the influence ofthe ions on the mobility and an electrostatic screening effect. © 2011 American Institute of Physics.#doi:10.1063/1.3581882$

Because of their potential as disposable sensors in healthcare and food monitoring applications, organic materialshave recently been studied as the active element in chemicalor biological sensors.1–5 It was shown that, even if the or-ganic layer is in direct contact with an electrolyte, organicfield effect transistors !OFETs" are stable and very sensitiveto analytes in the media.4,6 However, it is crucial that thesedevices can be operated at low voltages due to limitationsimposed by the electrolytic environment. To this end, high-kdielectrics,7 ultrathin organic layers,8 or electrolytes9–11 havebeen investigated as substitutes for the conventional oxidegate insulators.

The high electrical double layer !EDL" capacitanceformed at an electrolyte/organic semiconductor interface en-ables the operation of devices below 1 V. This working prin-ciple is also commonly used in solution-gated field-effecttransistors !SGFETs" with inorganic semiconductors likecarbon nanotubes,12 diamond,13 and graphene.14 In all thesecases the active material is inert at the potentials appliedacross the electrolyte/semiconductor interface. In the case oforganic semiconductors, electrochemical doping can lead toa modulation of the bulk conductivity of the organic film.2

However, modulation of the conductivity via a pure electricfield effect is favorable under certain conditions, as recentlydemonstrated for rubrene single crystals and spin-coatedpoly!3-hexylthiophene" films using pure water as theelectrolyte.9 In contrast to the chemical doping, the electricalfield across the EDL modulates only the conductivity in closeproximity to the interface with the electrolyte. Such devicescan be used to detect changes in the chemical and chargecomposition of the electrolyte, enabling chemical, and bio-sensing. In order to investigate the suitability of organicsemiconductors for SGFETs, we chose !-sexithiophene!!6T" since it was previously shown to be stable in an aque-ous environment.6

The !6T SGFETs were fabricated on oxygen-terminateddiamond substrates !Diamond Detectors Ltd" with a rough-ness below 0.2 nm in order to ensure good crystal growthand avoid any parasitic Faradaic currents from the substrate.Transistors with channel dimensions !length"width" of 20"200 #m2 were defined using conventional photolithogra-phy, followed by the thermal evaporation of Ti/Au !20 nm/

200 nm" contacts. Prior to the deposition of the organic semi-conductor, the contacts were covered with a chemicallystable photoresist !SU-8", leaving only a 20 #m long area ofgold exposed at the channel #see Fig. 1!a"$. 80 nm thin !6Tfilms !Sigma Aldrich" were deposited at a backgroundpressure of around 10!8 mbar with a deposition rate of 0.6Å/min and a substrate temperature of 45 °C. For measure-ments, the samples were mounted onto a ceramic chip carrierand all remaining exposed metal parts were covered withsilicone rubber. Unless otherwise specified, all measurementswere performed in an aqueous electrolyte containing 10 mMK-based phosphate buffer saline !PBS", adjusted to an ionicstrength of 50 mM with KCl. The gate voltage Usg was ap-plied between a Ag/AgCl reference electrode and the sourcecontact of the transistor. In the following, Usg will always begiven with respect to the Ag/AgCl reference electrode.

Prior to the transistor characterization, the !6T/electrolyte interface was investigated by electrochemical im-pedance spectroscopy in a three-electrode configuration. Forthis purpose, !6T was evaporated onto a polycrystalline goldelectrode. As a reference, a bare gold electrode was charac-terized. Figure 1!b" shows the frequency dependence of theeffective capacitance and the phase of the impedance. Theeffective capacitance was calculated assuming a simpleequivalent circuit, consisting of a capacitor in parallel with aresistor. As indicated by a phase around 80°, the impedanceis mainly of capacitive nature for frequencies below 103 Hz.The measured double-layer capacitances, CAu%20 #F /cm2

for the Au-reference and C6T=2–8 #F /cm2 for !6T, agreewell with literature values for gold and hydrophobic semi-conductors in contact with aqueous electrolytes.15,16 The

a"Electronic mail: [email protected].

(b)(a)

Uds

Ag/AgClreference electrode

Ti/Au contacts

resist

substrate

sexithiophene

electrolyte

Usg

10-1 101 103 10510-7

10-6

10-5

Ceff

(F/c

m2)

Frequency (Hz)

-80

-60

-40

-20

0

Ph

ase

(°)

Au reference

6T on Au

FIG. 1. !a" Transistor layout. !a" Effective capacitance extracted from im-pedance spectroscopy measurements on an electrolyte/!6T/Au stack com-pared to the capacitance of an electrolyte/Au reference. The frequency-dependent phase of the impedance is also shown.

APPLIED PHYSICS LETTERS 98, 153302 !2011"

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high capacitance of the EDL results in a large transconduc-tance and, thus, enables the operation of the device at lowvoltages. The capacitive behavior at low frequencies wasconfirmed by cyclic voltammetry measurements !data notshown", which exhibit no Faradaic peaks in the potentialregion below 0.6 V versus Ag/AgCl. Thus, electrochemicaloxidation of the organic semiconductor can be excluded atthese voltages. The literature value for the oxidation of solidfilms of !6T is 0.85 V versus Ag/AgCl.17

The characteristics of the device, as shown in Fig. 2!a",exhibit the typical behavior of a FET, with a clear saturationregime at higher drain-source voltages and a linear region,where the drain-source current can be described by the equa-tion

Idslin =

"CWL

#!Usg ! Ut"Uds !12

Uds2 $ , !1"

where " is the field-effect mobility of carriers in the thinfilm, Uds the drain-source voltage, Ut the threshold voltage,and C the double layer capacitance. Using Eq. !1", we canestimate field-effect mobilities of up to 2.2#10!2 cm2 /V sfor our devices. Roberts et al.6 reported a mobility of 1.8#10!1 cm2 /V s for bottom-gated !6T transistors immersedin water. This difference might be caused by the rougher!6T/electrolyte interface at which the conductive channel isformed in our case. The comparably low mobility was alsoobserved for water-gated devices in the work of Kergoatet al.9

Even though the organic film was directly exposed to theelectrolyte, the devices are quite stable, as shown in Fig.2!b". The drain-source current decreased by roughly 10%during continuous cycling of the source-gate voltage formore than 3 h, which is enough for the envisioned applica-tion as disposable sensing devices.

Figure 3!a" shows the dependence of the drain-sourcecurrent on the electrolyte pH, revealing a decrease in Ids withdecreasing pH. No sign of hysteresis was observed whensubsequently increasing the pH again. At pH values higherthan 7 the devices are unstable, the reason of which is cur-rently under investigation. This experiment was conducted ina 10 mM PBS-buffered solution with the ionic strength ad-justed to 100 mM with KCl. This high ionic strength was atleast one order of magnitude higher than any change in ionicstrength due to the addition of the 0.2 M HCl or KOH solu-tions used to adjust the pH. The resulting pH sensitivity!change in gate voltage against pH" was around 9 mV/pHversus Ag/AgCl. As shown in Fig. 3!b" this decrease in Ids is

due to a shift in threshold voltage and not due to a decreasingmobility, which would result in a variation in the slope of theIds!Usg curves. This can be interpreted considering the effectof a pH-dependent surface charge, rather than by the intro-duction of traps due to diffusion of hydronium or hydroxideions into the grain boundaries. The latter mechanism wouldhave an influence on the mobility since the transport inOFETs depends on the distribution of traps at the grainboundaries.18 The change in surface charge occurs either dueto protonation or deprotonation of groups at the surface, forexample, at the sulfur atom,19 or due to the specific adsorp-tion of OH! or H3O+ ions onto the surface. Interestingly, asimilar low pH sensitivity of 15 mV/pH was reported fordiamond devices and attributed to preferential adsorption ofwater ions.20The pH-dependent surface charge of conductiveelectrodes in an electrolyte has previously been describedwith the so-called amphifunctional model.21 In this model,the surface charge is balanced by the electronic charge in theactive layer and the diffuse charge as described by the Gra-hame equation.15 This can be idealized by three planar sur-faces, forming the inner layer capacitance C0 and the Helm-holtz capacitance C1. The applied potential drops over thesetwo capacitances and the diffuse layer. In the original model,the surface charge results from the protonation or deprotona-tion of amphoteric groups at the surface; however, the pres-ence of such groups at the !6T layer is not evident.Themathematical treatment of the specific adsorption of the dis-sociated water ions is the same. Furthermore, the preferentialabsorption of hydroxide ions, giving rise to the requirednegatively charged surface, is commonly observed on or-ganic surfaces.22 The parameters used for the simulation aregiven in the caption of Fig. 3. A fit to our data using thismodel is given by the solid line in Fig. 3!a".

In order to assess the sensitivity of the device to theionic strength of the electrolyte, the drain-source current wasrecorded while the salt concentration of the electrolyte wasincreased stepwise. During the whole measurement, the pHwas kept constant at 7 with the help of a 5 mM 4-!2-hydroxyethyl"-1-piperazineethanesulfonic acid !HEPES"buffer. Figure 4!a" summarizes the results of such measure-ments for different monovalent salts. After the measure-ments, the original conductance could be recovered by rins-ing the device with de-ionized water. As can be seen in Fig.

(a) (b)

0 1 60 120 180

0

5

10

15

20

I ds/I

ds(o

ff)

Time (min)

Uds=-0.4V

0.0 -0.2 -0.4 -0.60

-10

-20

-30

I ds

(nA

)

Uds (V)

Usg= 0.3 V to 0.6 V

FIG. 2. !a" Transistor characteristics of an electrolyte-gated !6T OFET. !b"Recording of the drain-current during continuous cycling of the device be-tween its on !Usg=0.6 V"- and off-state !Usg=0.3 V".

(b)(a)

0.3 0.4 0.5 0.6 0.70

-5

-10

pH2

pH5

I ds

(nA

)

Usg(V)

Uds= -0.1 VpH7

2 4 60.4

0.6

0.8

1.0

1.2

Uds= -0.1 V

Usg= 0.6 V

I ds/

I ds(p

H=

5.4

)

pH

FIG. 3. !a" Dependence of the drain-source current on the pH of theelectrolyte measured for decreasing !solid symbols" and increasing pH!open squares". The solid line shows the current simulated using the amphi-functional model with the following parameters: C0=2 "F /cm2, C1=20 "F /cm2, the surface site density used is 3#1013 cm!2. The acidityand alkalinity constants were 1#10!4 and 1#10!3, respectively. !b" Draincurrent vs gate voltage measured in electrolytic solutions with different pH.The lines show fits to the linear region.

153302-2 Buth et al. Appl. Phys. Lett. 98, 153302 !2011"

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The variation of the threshold voltage is attributed to the variation of the surface charge with pH

Buth, F. et al. Appl. Phys. Lett. (2011) 98 153302


EGOET biosensor: penicilin

25

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chemically stable photoresist in order to minimize the exposure of the metal to the electrolyte. Only in the channel region, a 5 µ m part of the metal is still exposed and serves as the contact to the 100 nm thin polycrystalline ! 6T fi lm (see the schematic in Figure 1 a and the experimental section for further informa-tion). A Ag/AgCl reference electrode immersed in the electro-lyte is used to modulate the channel conductivity by applying a voltage between the electrode and the source contact and thereby shifting the Fermi level in the organic semiconductor. As previously shown, [ 13 ] this confi guration ensures fi eld-effect operation without signifi cant charge transfer between the elec-trolyte and the organic semiconductor in the potential window of the organic layer.

The electrical double layer induced at the interface between the aqueous electrolyte and the organic semiconductor leads to the formation of an interfacial capacitance with values of around 2 µ Fcm " 2 . As a result, the output transistor charac-teristics (see Figure 1 c and 1 d) show the typical behavior of a fi eld-effect transistor already at gate voltages below 1 V. The on/off ratio at U ds = –50 mV is of the order of 10 2 –10 3 and the extracted fi eld-effect mobility reaches up to 4 # 10 " 2 cm 2 V " 1 s " 1 . This mobility is about an order of magnitude lower as has been reported for back-gated devices operated in aqueous environ-ment, which might be due to the rough interface between the organic layer and the electrolyte in our case, as compared to the smooth surface of the polymer dielectric in the work of Roberts et al. [ 19 ] All these values are deduced from transistor measure-ments in a 10 m M phosphate-buffered saline (PBS) solution, with a pH of 5 and its total ionic strength adjusted to 50 m M

by KCl. It is worth to recall that in these devices the organic semiconductor is in direct contact with the electrolyte and no passivation layer is used. Even under these conditions, we have previously found that the devices are stable during continuous operation in the electrolyte. [ 13 ] To enhance the lifetime of the devices, a proper selection of the bias operation within the elec-trochemical potential window is crucial.

A typical limitation to the stability of organic thin fi lm transistors is the oxidation of the material upon exposure to atmospheric oxygen or ozone in combination with UV illu-mination. [ 20 , 21 ] We used this mechanism in order to introduce oxygen-related species at the surface, which will be later used to control the chemistry and sensitivity response of the organic FET sensors. To this end, the samples were kept in a closed container under a pure oxygen atmosphere and then illumi-nated for varying durations with an UV lamp. As a result, the characteristics of the transistor were affected as follows: upon exposure to the oxidizing environment we observed a decrease in the transconductance and a shift of the threshold voltage U th towards more negative gate voltages (see supporting informa-tion and the blue dots in Figure 1 d). This effect is tentatively attributed to the formation of oxygen-related trap states at the interface, which are expected to decrease the effective mobility in accordance with the multiple trap and release model. [ 22 ] In addition, the oxygen-related surface moieties are positively charged and thereby shift U th to more negative values.

A similar behavior can be observed for APTES modifi ed tran-sistors (green dots in Figure 1 d). The surface functionalization was conducted by exposing the untreated ! 6T fi lms to APTES

Figure 1 . a) Schematic drawing of the transistor layout, including the different functionalization steps investigated in this work. b) Static water contact angle measurements of differently treated ! -sexithiophene thin fi lms. c) Typical output characteristic of a transistor with a width-to-length ratio of 4900 recorded at pH 5. d) Transfer curves of untreated, oxidized (5 min UV illumination) and APTES-functionalized transistors, measured at pH 5.

Adv. Mater. 2012, 24, 4511–4517

(penicilase)

4514

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variations of the pH with a shift in their threshold voltage U th , while the mobility remains constant. [ 13 ] Therefore, the shift of U th can be directly measured in the constant source-drain cur-rent ( I ds ) mode, in which the transistor is biased with a con-tinuous source-drain voltage while a feedback loop controls the gate voltage in order to keep I ds constant. Figure 3 a summarizes the pH sensitivity of unmodifi ed, UV-oxidized, and APTES-func-tionalized organic SGFETs. According to this fi gure, all samples exhibit a negative shift of the threshold voltage when going from neutral towards acidic electrolyte conditions. For untreated ! 6T transistors, a sensitivity of 14 mV pH " 1 is obtained up to a pH of 5, decreasing to around 10 mV pH " 1 for more neutral solu-tions. Upon oxidation (300 s) the sensitivity in the region below pH 5 increases to 26 mV pH " 1 , while it remains unchanged for higher pH values. A similar behavior can be observed for the transistors functionalized with APTES. In this case, however, the sensitivity increases to 21 mV pH " 1 in the neutral region, while no changes are visible for more acidic electrolyte condi-tions. Figure 3 b depicts the threshold voltage shift for the two modifi ed surfaces (UV oxidation and APTES-functionalized) for which the response of the untreated ! 6T fi lm were subtracted,

an APTES fi lm thickness of around 1 nm, which agrees well with the theoretical value of a monolayer of 8.5 Å. However, this value should be taken with care due to uncertainties in the attenuation length and the azimuth angle. [ 27 ] In summary, the XPS data confi rms the successful introduction of amino- or oxygen-related groups to the surface after the respective func-tionalization steps.

Figure 2 d shows an atomic force microscopy (AFM) image of an untreated ! 6T fi lm, featuring the typical thin fi lm mor-phology of ! 6T layers with the expected step height of around 2.2 nm. [ 28 ] The fi lm morphology is not substantially altered upon the functionalization with APTES, as can be seen in the right side of the image. Only small features are visible on the fi lm, which might stem from local multilayer formation on certain spots. This is not surprising considering that harsh cleaning methods usually employed to remove any physisorbed species cannot be used for organic thin fi lms without disrupting their fi lm quality.

Titration experiments were performed in order to assess the effect of the surface modifi cation on the pH sensitivity of the ! 6T SGFETs. As previously reported, the transistors respond to

Figure 3 . a) Threshold voltage shift of untreated ! 6T fi lms, APTES-functionalized and UV-oxidized samples upon a change in the pH of the electrolyte, obtained by titration measurements. The shift is calculated with respect to the threshold voltage at pH 5. The dashed lines correspond to fi ts using the amphifunctional model (see text). b) The graph depicts the threshold voltage shift of the APTES-treated sample and the oxidized samples after the subtraction of the response of untreated ! 6T. The solid lines are fi ts to the data. c) Time-lapse of I ds during a penicillin titration experiment conducted in a 0.5 m M PBS buffer. The ionic strength was adjusted to 50 m M using NaCl. The green line corresponds to the response of a device functional-ized with APTES and penicillinase, while the grey line depicts the I ds response of an untreated ! 6T OFET. The pH of the electrolyte is shown in red. d) Plot of the calculated threshold voltage shift due to a change in penicillin concentration of the electrolyte for as-prepared devices with physisorbed enzymes and chemisorbed enzymes and after three washing steps. The dashed line shows the simulated response due to pH variations in an unbuff-ered electrolyte.

Adv. Mater. 2012, 24, 4511–4517

Penicilase catalyses the decomposition of pénicillin into penicilloic acid and one proton, which is in turn detected by the pH sensor

Buth, F. et al., Adv. Mater. (2012) 24 4511


Bioelectronics

26

❖Organic transistors (and their electrochemical variants) can also be used in bio-devices other than biosensors


Ionic pump

27

4.1. Principles of the organic electronic ion pump

Recently, the “organic electronic ion pump” (OEIP) was developedto provide a device for lateral transport and electronically controlleddelivery of ions and other positively charged biomolecules [57]. TheOEIP is produced from a single film of the conductive polymer PEDOT:PSS patterned into two electrodes joined by a “transport channel”(Fig. 7A). Over-oxidation of the PEDOT:PSS in the channel deactivatesthe electronic conduction while preserving ionic conduction. Ahydrophobic resist covers the channel and provides openings toposition the electrolytes on top of the electrode areas (Fig. 7B). Byapplying a voltage between the electrodes an electrochemical circuitis established. Oxidation of PEDOT occurs at the anode/source(Eq. (1)) whereas reduction occurs at the cathode/target (Eq. (2))according to the half reactions:

PEDOT0 + M! : PSS– ! PEDOT! : PSS– + M! + e– "1#

PEDOT! : PSS– + M! + e– ! PEDOT0 + M! : PSS– "2#

Together, these reactions drive the migration of positively chargedspecies (M+) from the source electrolyte into the polymer (anode).The M+ is then transported through the charged field in the channeltowards the target electrode, where they are released into the targetelectrolyte. Migration of ions in the channel resembles the transport ofcharged molecules, such as DNA and proteins, in conventional gelelectrophoresis systems. In gel electrophoresis, the motion of chargedspecies through an electrically chargedfield in the gelmatrix separatesmolecules according to their charge and size. In analogy to this, theOEIP offers electrophoretic transport and delivery of positivelycharged ions and biomolecules in the absence of a liquid flow.

The electrochemical relationship of Eqs. (1) and (2) ensures thatthe delivery rate of positively charged species into the target system isdirectly proportional to the current, which can be measured in thecircuit. The OEIP functionality was first demonstrated and character-ized for transport of K+ (Fig. 8A), whereas later studies revealed that

transport functionalities of the OEIP could be expanded to include anumber of small positively charged bio-molecules, primarily belong-ing to the category of small-molecule neurotransmitters. Table 1 liststhe transportable species and their selected biological functions.

Fig. 7. (A) Geometry of the OEIP. Arrow indicates the flow of cations. S=sourceelectrolyte; T=target electrolyte. (B) Photograph of a 4-mm channel OEIP transportingK+. The redox-reaction of the electrodes is observed by the color change: Figure A isreproduced from [67].

Fig. 8. (A) Relationship between K+ delivered to the target system after 10 min and thetotal charge measured in the electronic circuit (i.e. integrated current) for drivingvoltages set to 2 V, 5 V, and 10 V. The middle line shows the linear fit and the two outerlines represent the 95% confidence interval. (B) Intracellular Ca2+ recordings of HCN-2cells by delivery of K+ in the absence (dashed line), or presence (solid and dotted lines)of pharmacological substances that block voltage-operated Ca2+ channels. Figuresreproduced from [57].

Table 1Ions and neurotransmitters transported in the OEIP.

Transportedspecies

Selected functions Refs

Protons and metal ionsH+ Establishes pH gradients, involved in the electron

transport chain.[60,61]

Na+ Maintains the osmotic balance, involved in theinitiation of action potentials.

[57,69]

K+ Maintaining the resting potential in excitable cells. [57,69]Ca2+ Involved in cell signaling, dictate e.g.

neurotransmitter release.[57,62]

NeurotransmittersAcetylcholine Major neurotransmitter in peripheral and

central nervous systems[59,69]

Glutamat Major excitatory neurotransmitter in the brain,involved in memory and learning.

[67,70]

Aspartat Excitatory amino acid in the brain [67,69]GABA Major inhibitory neurotransmitter in the brain [67,69]

281K. Svennersten et al. / Biochimica et Biophysica Acta 1810 (2011) 276–285

NATURE MATERIALS DOI: 10.1038/NMAT2494LETTERS

Asp is also an excitatory neurotransmitter, whereas GABA is theprincipal inhibitory neurotransmitter in the mammalian CNS.The pathophysiology of numerous neuropsychiatric disorders,including anxiety and depression, is suggested to be due todisturbances in the GABA system25.

We prepared planar ion pump devices, comprising a single,biocompatible PEDOT:PSS layer, as previously reported18. Thesingle film is divided by an electronically insulating—but still ioni-cally conducting—region (Fig. 1 and Supplementary Information).When voltage is applied, an electrochemical circuit is established,leading to oxidation of the source electrode (anode, equation (1)),and reduction of the target electrode (cathode, equation (2)), whereM+ is the cation present in the source electrolyte.

PEDOT0 +M+:PSS� !PEDOT+:PSS� +M+ +e� (1)

PEDOT+:PSS� +M+ +e� !PEDOT0 +M+:PSS� (2)

The cations are electrophoretically transported through the regionof film joining the two electrodes, then enter the cathodic sideof the film, where they are delivered into the electrolyte. Thisstructure of electronically conducting electrodes separated by anelectronically insulating channel enables application of a wide rangeof voltages—in excess of 30V—without excessive electric fields inthe target system. Furthermore, the use of PEDOT:PSS alleviatesproblems associated with secondary electrochemical reactions atsuch elevated voltages (see Supplementary Information).

Using Glu, Asp and GABA as source electrolytes, the deliverycapabilities of the device were demonstrated at multiple drivingvoltages for a variety of times, with individual devices used foreach parameter tested (Fig. 2a–c). By comparing the integratedelectronic current to the quantity of neurotransmitters deliveredinto the target electrolyte (see Supplementary Information), atransport efficiency can be defined by the electron/molecule ratio.This ratio is precisely 2.7 ± 0.2 for Glu (n = 16), 6.3 ± 0.5 forAsp (n= 10) and 1.3± 0.1 for GABA (n= 10) (value± s.d.). Formaterials with low pKa, that is, Glu and Asp, the excess protonspresent in the source solution will also be pumped. Owing to thesmaller size of protons compared with Glu or Asp, their mobilitythrough the channel can be significantly higher, explaining thelarger electron/molecule ratios for Glu and Asp. The transportrate can be tuned by the operating voltage, providing a fullrange of transport up to quantities of the order of 100 µM inthe total liquid volume. Locally however, the concentration canbe significantly higher before the molecules diffuse away23. Theapproximate concentration of Glu in the synaptic cleft has beenreported to be 2–1,000 µM (ref. 26); thus, the device operates in arelevant physiological range.

The lifetime of the device is limited by the amount ofoxidizable PEDOT (see equation (1)) and can thus be tailored bythe electrode dimensions (see Supplementary Information). Thepresent electrode geometry enables device operation of the orderof 1 h in constant pumping mode, that is, constant current. ThePEDOT could also be returned to amore reduced state, for example,by reversing the voltage. Using pulsed delivery23, the lifetime shouldbe bounded only by the concentration of molecules in the sourceelectrolyte and could therefore be markedly extended. Figure 2dillustrates that the delivery rate is stable after an initial equilibrationperiod corresponding to the time required to fill the channel withthe intended ions on first use.

The successful transport of neurotransmitters encouraged us toredesign the device into an encapsulated, syringe-like form to enableits use in vivo. The first step in this development was the realizationthat the channel region could comprise an extra electrolyte(Fig. 1b). This geometry could enable the central electrolyte to

TCathode

Anode, S

Anode CathodeT

S

Anode Cathode

S T

A

Hydrophobic encapsulation (10 µm thick)

Over-oxidized PEDOT:PSS channel (250 nm thick)

PET substrate (~150 µm thick)

PEDOT:PSS (250 nm thick)

2 mm

30 mm

~6 mm~3.4 mm

0.75 mm

7 mma

b

c

d

T

S reservoiranode

Figure 1 | Planar and encapsulated geometries of the delivery device.a, Side view of the planar device used in initial Glu, Asp and GABA transportstudies. The black arrow indicates the flow of charged neurotransmittersfrom the source electrolyte, S, through the anode, then through theover-oxidized channel and finally out into the target electrolyte, T, throughthe cathode. Layer thicknesses are indicated in the material legend. b, Sideview showing the developmental progression from the planar device (a), tothe planar device with intermediate electrolyte (salt bridge)—that is, theeffective target system T. The white arrow indicates the flow of arbitrarypositively charged species from T into the cathodic electrolyte. c, Side-viewscheme of the encapsulated device. Only the source/anode system isshown with cylindrical encapsulation. The arrow again indicates ion flow.d, Top view of the encapsulated device, showing both electrolyte chambersand the requisite target system T. The arrows are analogous to those in b.The electrolyte reservoir tubes are 2 mm in outer diameter.

become the effective target system, with the original target actingsimply as the electrochemical cathode. By ‘folding’ this system,and providing encapsulation around the individual electrolytes, asyringe-like device is achieved (Fig. 1c,d). The operating principle issimilar to the planar device, except that cations from the source aredelivered to an external target electrolyte, and cations are extractedfrom this target region and drawn in towards the cathode system,completing the electrochemical circuit.

To ascertain whether the encapsulated device can be used forcell stimulation similarly to the planar device18, in vitro experimentswere carried out. Astrocytes, a sub-type of glial cells present in theCNS, express the receptor for Glu. On binding of Glu, membrane-bound ion channels open immediately, promoting Ca2+ influx27–29.Therefore, astrocytes represent an ideal system to monitor Gluactivation of cells using real-time single-cell ratiometric Ca2+imaging. The device was loaded with Glu (source) and NaCl(cathodic) electrolytes andmounted with the tip in contact with thebottomof a dish, adjacent to primarymurine astrocytes. After initialbaseline recordings of intracellular Ca2+ with the delivery devicein the off-state, Glu transport was activated. A significant increase

NATUREMATERIALS | VOL 8 | SEPTEMBER 2009 | www.nature.com/naturematerials 743


Drug delivery

28

5.3. Generation of oscillating Ca2+ signals

The Ca2+ ion is one of the most versatile signalling agents.Research over the last decades has revealed that cells use repetitivesignals, known as Ca2+ oscillations, when information must berelayed over longer time periods [62,63]. The switchable nature ofion delivery in the OEIP provides a novel tool to decipher the role ofthe frequency and amplitude components of the Ca2+ oscillations.Due to the electronic control of the OEIP, switching the device ON/OFFwill result in a similar ON/OFF behaviour of the electrophoreticdelivery. This strategy can accordingly be used to establish temporallycontrolled oscillations.

Fig. 9B shows a typical example of OEIP-induced Ca2+ oscillation incells. By pulsing the delivery of ACh using the 10 μm-channel OEIP, arepetitive Ca2+ responsewas induced in neuroblastoma cells [59]. Thehigh spatial resolution of the ACh pulse is demonstrated, since onlythe cell located circa 50 μm from the channel outlet is activatedwhereas the cell located 100 μm further away is not (see arrowheadsin Fig. 9A). It can also be noted that the amplitude of the Ca2+

response can be modulated. This can be achieved by increasing thetime the device is kept in its ON state, i.e., the time voltage is applied,exemplified by the short high-potential pulses ranging from 0.2 to2.0 s. Alternatively, the amplitude of cell responses can be increasedby applying a higher voltage, generating a faster delivery of ACh, for adefined pulse length.

The precise, electronically controlled, lateral transport in the OEIP,and the corresponding substance delivery, provides a new techniqueto stimulate cells in the absence of any liquid flow. Control of dynamicparameters, such as frequency and amplitude, is not limited to ACh,but has also been demonstrated using H+ [61].

6. Biosubstance release for in vivo applications

Stimulating neuronal activity in vivo is generally performed usingneural probes that generate electrical signals. Compared to chemicalmessengers, electrical signals cannot discriminate amongst the targetcells, as all excitable cells in the stimulated area will be affected. Atechnique allowing local stimulation using a specific chemicalcompound is highly desirable, and this need has sparked greatactivities in the area of drug delivery systems. Delivery systems areoften classified as “active” or “passive”. A typical example of an activesystem is microfluidics. These systems are liquid-phase based,generally small, programmable and self-contained devices forsustained, local release [64]. Hydrogels, on the other hand representsa passive system, made of cross-linked water-soluble polymers [65].These systems are commonly used in clinical practice for wound-healing and tissue regeneration.

The above-mentioned techniques have been applied in inner eardiseases and disorders, where local delivery has been a long-standingchallenge [66]. The cochlea is a complex structure of the inner ear thatconverts sound into neural activity. The coiled liquid-filled tubeincludes a membrane stretched across the middle of the tube andcovered with sensor cells (hair cells) detecting sound using theirmechanotransduction apparatus. Because these elements are excep-tionally force- and flow-sensitive, and since the cochlea contains onlya limited volume of liquid (so called perilymph) local delivery to thissite is exceptionally challenging.

As substances are transported in the OEIP via electrophoretictransport rather than liquid flow, this device seemed very attractive touse for local delivery in vivo in general and the cochlea in particular.The OEIP was therefore re-designed from a planar structure to adopt amore syringe-like, flexible device. This was accomplished by “folding”the device in half, which separated the two encapsulated electrode-electrolyte systems (Fig. 10A) [67]. Using this design, the channeloutlet comes in direct contact with the target system; in this case thefluid covering the round windowmembrane of the cochlea (Fig. 10B).

After filling the tube containing the anode with the substance-to-be-delivered, and the cathode-containing tube with a counter electrolyte,a voltage is applied from a power supply. This activates theelectrophoretic transport of the signal substance, which is deliveredto the cells in the absence of any liquid flow.

6.1. Organic bioelectronics modulate mammalian sensory systems in vivo

Guinea pig is a preferred animal model for studying the auditorysystem. The structure of their ear, as well as the range of hearingfrequencies, is very similar to that of humans. In both models, theinner and outer hair cells of the cochlea respond to mechanicalstimulation from sound waves. The inner hair cells are the principalcells responsible for converting the stimulus into neural activity,thereby giving rise to the brain's perception of sound. However, thevolume of the perilymph in scala tympani of guinea pigs is small, only~8 μl compared to ~30 μl in humans [68]. This makes local delivery ofany substance a challenging task.

To test the feasibility of using the OEIP as a delivery device in vivo,the OEIP was positioned at the round window membrane. The devicewas activated by a constant voltage for 1 hr, thereby deliveringglutamate (Glu) to the RWM,which is an established point of diffusiveaccess to the perilymph (Fig. 10B, C). The inner hair cells utilize Glu asthe primary neurotransmitter and accordingly, they express the Glureceptors. As Glu diffuse up the cochlea, from the base to the apicalparts, Glu-receptors become over-activated. Due to this excitotoxiceffect of Glu on the inner hair cells, loss of hearing sensitivity fromhigher to lower frequencies is expected. On the contrary, the outerhair cells, which do not express Glu receptors, should not be affected.By monitoring the auditory brainstem responses (ABR), the hearing

Fig. 10. (A) Top view of the syringe-like OEIP. S=source electrolyte; T=targetelectrolyte. (B) Cartoon depicting the experimental set-up in vivo. Dotted line indicatesa slice through the cochlea, expanded below. Grey arrow=diffusion of ions through theRWM. (C) Devicemounted on the RWM in a guinea pig. The two ion channels are visibleas dark blue stripes on the transparent substrate. (D) Histological analysis shows thatinner hair cells, as opposed to outer hair cells, are affected (*=dendrite damage).Figures reproduced from [67].

283K. Svennersten et al. / Biochimica et Biophysica Acta 1810 (2011) 276–285

Delivery of glutamate in the internal ear of a guinea pig

Main interest: Ionic transport without fluid transport

Simon, D. T. et al. Nature Mater. 2009 8 742

LETTERS

NATURE MATERIALS DOI: 10.1038/NMAT2494

NH3+

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ol s¬1)

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Figure 2 | Transport of neurotransmitters in the planar delivery device. a–c, Concentration of Glu (148 g mol�1) (a), Asp (134 g mol�1) (b) and GABA(104 g mol�1) (c) in the target electrolyte (150 µl) as a function of time, where the device was operated at 4 V (open symbols) and 8 V (filled symbols). Thegrey lines are linear fits with non-zero time offset (see Supplementary Information). The insets show the chemical structures of the materials in theirpositively charged form. d, Representative current versus time data at 4 V (dashed line) and 8 V (solid line) with equivalent delivery rate on the right axis.The data shown are for GABA delivery.

1.0

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)

Figure 3 | Glu-induced Ca2+ responses in astrocytes. Time-lapsefluorescence microscopy of intracellular Ca2+ recordings in astrocytesinduced by Glu delivered by the encapsulated device. Representativetracings from three cells (solid, dotted and dot–dash lines) in oneexperiment are shown. The device was switched on by application ofvoltage for 370 s (solid grey line) and then switched off. Cells were thenallowed to re-establish their basal Ca2+ level. At 730 s, Glu was appliedmanually using a pipette ([Glu]final = 1 mM), eliciting responses of similarmagnitude to those from the device-delivered Glu. The experiment wasrepeated three times.

in intracellular Ca2+ revealed successful delivery of Glu (Fig. 3)after an equilibration delay similar to that observed in Fig. 2d. TheCa2+ response declined when the voltage was turned off and cellsre-established their basal intracellular Ca2+ level. As a control, Gluwas next applied manually using a pipette, resulting in a Ca2+response of similar magnitude to that induced by Glu deliveredfrom the encapsulated device. Collectively, this substantiates theusability of the device for cell activation, and that Glu retains itspotent biological form after transport through the polymer.

Having demonstrated how this device converts electronicaddressing signals into precise non-convective delivery of Glu, wenext investigated its potential use as a novel communication inter-face between human-made electronics, selective neurotransmission

and brain function. To test the feasibility of this concept, we usedthe auditory system of the guinea pig as an in vivo experimentalplatform. Within the cochlea, sound waves of various frequenciesare transduced primarily by the inner hair cell system, as opposed tothe outer hair cell system30. As Glu is the primary neurotransmitterfor the inner hair cells, the auditory system can be used to evaluatethe ability of the device to target specific cells, that is, whether theencapsulated ion pump can be used to selectively affect specificcell types in vivo. Excessive Glu is known to exert an excitotoxiceffect on the inner hair cells as opposed to the outer hair cells31,32,and this pathophysiological effect can be monitored by histologicalanalysis. In addition, excitotoxicity can be analysed in real-time,by monitoring the auditory brainstem response (ABR). ExcessiveGlu will damage the inner hair cells, leading to hearing loss andan attenuated ABR. In the cochlea, high-frequency sound wavesare transduced at the base, near the round window membrane(RWM), whereas lower frequencies are transduced towards theapex. Therefore, shifts in ABR threshold (re:pre-treatment thresh-old) at different frequencies indicate, in real-time, the effect of Gluat different distances up the cochlea. At present, osmotic pumps areused to modulate such effects by direct fluid injection33. However,the inner hair cells, positioned on a delicate membrane that vibrateswith sound stimulation, are highly mechanosensitive, for example,to disturbances in fluid flow and pressure34. To bypass this problem,the ion pump can be used to deliver substances through the RWM,which is an established port of diffusive entry into the cochlea.

The tip of the delivery device was introduced in close proximityto the RWM of anaesthetized guinea pigs using conventionalontological surgery (Fig. 4a). The procedure is non-invasive to thecochlea, because the device delivers Glu to the outside of the RWMthrough which Glu enters by diffusion (Fig. 4b). Once the devicewas in place, Glu was delivered continuously for 60min (n = 5)and during this time, ABR shifts were assessed at 0, 15, 30 and60min. Within this time frame, stimulation in the lower basalregion (corresponding to 20 kHz transduction), upper basal region(16 kHz) and lower apical region (8 kHz) could be achieved withoutrisking saturation of the entire cochlea withGlu. As theGlu solution

744 NATUREMATERIALS | VOL 8 | SEPTEMBER 2009 | www.nature.com/naturematerials

Glu


Measuring brain activity

29

❖Electroencephalography (EEG) measures brain activity by recording voltage fluctuations resulting from current flow in the neurons

❖The EEG is discribed in terms of of rhythmic activity divided in frequency bands ranging between 0.5 and 200 Hz

❖Some of these bands are typical of pathologies such as epilepsy


❖ In the treatment of epilepsy, it is important to locate the epilectic activity within the brain. This is done by implanting electrodes at the surface or inside the brain


«Crumply» OECT

Parylene (2 µm) Au (100 nm) PEDOT:PSS (200 nm)

D S

E

a

E S D

b

c d


OECT

Electrode

SNR = 52.7 dB

SNR = 30.2 dB


OECT at the surface

Electrode at the surface

Implanted electrode

G. G. Malliaras et al., Nature Comm. (2013) 4 1575

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